Products using piezoelectric ceramics include, for example, a piezoelectric actuator, a filter, a piezoelectric resonator (including an oscillator), an ultrasonic vibrator, an ultrasonic motor, a piezoelectric sensor, and the like. Amongst these products, the piezoelectric actuator has a response speed to an electrical signal of as fast as 10−6 seconds. The piezoelectric actuator is used to position an XY stage of a semiconductor manufacturing apparatus or for a printing head (liquid eject apparatus) of an ink jet printer. Recently, with high speed and low cost being required for a color printer, the demand for an ink eject piezoelectric actuator of an ink jet printer or the like has been increased.
JP-A-2004-165650 discloses a piezoelectric actuator that has a plurality of displacement elements each having a piezoelectric ceramic layer (piezoelectric layer) and a pair of electrodes with the piezoelectric layer interposed therebetween. The piezoelectric layer is disposed on a surface of a ceramic substrate. A piezoelectric vibrating plate formed of a piezoelectric material (piezoelectric ceramics) is used as the ceramic substrate, and the piezoelectric actuator is used for a printing head (liquid eject apparatus).
FIG. 5 is a schematic cross-sectional view showing a known liquid eject apparatus having a piezoelectric actuator disclosed in JP-A-2004-165650. As shown in FIG. 5, the liquid eject apparatus 70 has a plurality of liquid flow passages 53a arranged in parallel. The liquid eject apparatus 70 also has a piezoelectric actuator 61 provided on a flow passage member 53 which forms partition walls 53b serving as walls partitioning the individual liquid flow passages 53a. 
The piezoelectric actuator 61 has a plurality of displacement elements 67 provided on a surface of a piezoelectric vibrating plate 62. Each of the displacement elements 67 has a piezoelectric layer 65 and a pair of electrodes (an internal electrode 64 and a surface electrode 66) with the piezoelectric layer 65 interposed therebetween. The piezoelectric actuator 61 is mounted on the flow passage member 53 such that the positions of the liquid flow passage 53a and the surface electrode 66 are aligned with each other.
A common liquid supply flow passage (not shown), which is provided in the flow passage member 53, is connected to each of the liquid flow passages 53a. The liquid from the outside is supplied to and filled into each of the liquid flow passages 53a from the liquid supply flow passage. If the liquid is ejected, the ejected liquid flows into each of the flow passages 53a through the supply flow passage. For this reason, the rear surface of the piezoelectric vibrating plate 62, which is located above each of the liquid flow passages 53a, is brought into contact with the liquid flowing in the liquid flow passage 53a. 
The liquid eject apparatus 70 displaces a displacement element 67 by applying the driving voltage between the surface electrode 66 and the internal electrode 64 so as to change a volume of the liquid flow passage 53a. Accordingly, the liquid in the flow passage 53a is pressurized and the liquid is ejected from the liquid eject port 58 that is opened at the bottom of the flow passage member 53.
However, if a predetermined driving voltage is repeatedly applied for a long time in a state where the piezoelectric vibrating plate 62 is brought into contact with the liquid, the internal electrode 64 and the piezoelectric vibrating plate 62 are separated from each other. Thus, driving durability is lowered. This problem drastically appears in a piezoelectric actuator having a thickness of not more than 100 μm.